Backupify has an interesting tool named Migrator for Google Apps. As the name suggests, this service enables you to move data between two Google Apps account.
Google does not offer such a service on their own and if you are an administrator of a Google Apps account, this can come very handy.
The company highlights some use cases of this service:
Company acquires another company where both are on Google Apps
User leaves a company, wants to transfer all info to another user
Business changes their domain name in rebranding or in a merger or spin-off
Schools move students from student to alumni domain
Company signed up with Google Apps using a test domain name and want to migrate to a more professional domain name